This invention relates to a method and to an apparatus for the production of composite sheet material and to a sheet material produced thereby.
The invention has been devised especially in relation to sheet material for use in formwork for casting concrete, as an alternative or substitute for the conventional thick plywood sheets used at present. These plywood sheets have the disadvantages that their life is relatively short, they are susceptible to swelling and splitting in the wet environment in which they are used, and the sheets need considerable support per unit area since wood is a relatively flexible material.
In view of these disadvantages of plywood sheets, it has been proposed to use sheets of a material comprising glass-fibre reinforced cement (grc). Such sheets have good release properties when used as temporary formwork, are extremely stiff and strong and hence need less support than plywood for an equivalent permitted deflection, and are less susceptible to water damage than plywood. The sheets can be formed so as to provide a flat or alternatively a relief pattern surface on the concrete being cast.
However, currently available grc sheets have certain disadvantages. They are much heavier than plywood of the same thickness. grc is relatively expensive because of the need to use an alkali resistant form of glass fibre in the mix to prevent attack by the cementitious matrix. At present, grc sheets are laid down by a spraying process which is inherently likely to produce a slightly uneven top surface finish and so that thickness of the sheets tends to vary and the surface finish is often not even. The top surface of material has been rolled by hand to consolidate the material and then floated to produce a more or less flat surface but, in spite of the costly and time consuming nature of such hand finishing processes, the resulting sheets are still slightly non-uniform. When used in formwork, sheets are supported by wooden frameworks and it has often proved necessary to pack and shim the formwork extensively to achieve a satisfactory structure.
The invention is not limited to the manufacture of sheet material for use in permanent or temporary formwork for concrete. It is also applicable to the manufacture of sheet material for other uses, for example as flooring or cladding or for other building purposes.